Multicore fiber and method for manufacturing the same

Abstract

To provide a non-coupling type multi-core fiber high in manufacturability and a method of manufacturing the same.SOLUTION: A multi-core fiber includes: a plurality of core glass bodies each having a core and an inner cladding surrounding an outer periphery of the core; and an outer cladding surrounding an outer periphery of the plurality of core glass bodies. The plurality of core glass bodies is arranged so that adjacent core glass bodies substantially in a line on a cross-section perpendicular to a longitudinal direction are in contact with each other. The refractive index of the outer cladding is lower than the maximum refractive index of the core, and an outer periphery portion of the inner cladding, which is adjacent to at least the outer cladding, has a lower refractive index than the outer cladding.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a multi-core fiber and a method for manufacturing the same.

Background Art

[0002] In recent years, due to advantages such as ease of arrangement, a single-row multi-core fiber in which cores are arranged in a row in a cross-section perpendicular to the longitudinal direction has been actively studied. However, when attempting to manufacture such a single-row multi-core fiber using a drilling method, holes corresponding to the number of cores must be drilled in the glass base material. Therefore, its manufacturing is relatively difficult.

[0003] In contrast, Patent Document 1 discloses a method for manufacturing a single-row multi-core fiber by arranging a plurality of optical fiber strands in a row in the grooves of a glass plate having wide grooves, covering the top with a glass plate and integrating them to form a base material.

[0004] Further, Patent Document 2 discloses a single-row multi-core fiber of a coupling type and a method for manufacturing the same. A coupling type multi-core fiber is a multi-core fiber in which a plurality of cores are optically coupled to each other and can be regarded as substantially one multi-mode transmission path.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In this context, there is room for improvement in terms of manufacturability for uncoupled, single-row multicore fibers, where the optical coupling between multiple cores is weak and they can be considered as independent transmission paths. In the case of uncoupled fibers, for example, the crosstalk between two adjacent cores (also called two-core crosstalk (XT)) is -20 dB or less over a length of 10 m.

[0007] The present invention has been made in view of the above, and its object is to provide a highly manufacturable uncoupled multicore fiber and a method for manufacturing the same. [Means for solving the problem]

[0008] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention is a multicore fiber comprising a plurality of core glass bodies each having a core and an inner cladding surrounding the outer periphery of the core, and an outer cladding surrounding the outer periphery of the plurality of core glass bodies, wherein the plurality of core glass bodies are arranged in substantially a single row in a cross section perpendicular to the longitudinal direction and adjacent core glass bodies are in contact with each other, the refractive index of the outer cladding is lower than the maximum refractive index of the core, and at least the outer periphery of the inner cladding adjacent to the outer cladding has a refractive index lower than that of the outer cladding.

[0009] At least a portion of the inner cladding may be made of fluorine-containing quartz glass.

[0010] The outer cladding may consist of pure quartz glass, quartz-based glass that does not contain intentionally added dopants, or quartz-based glass containing fluorine.

[0011] The core may be made of a silica-based glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium.

[0012] The propagation mode may be single-mode at the specified operating wavelength.

[0013] The core crosstalk at a length of 10 m at the specified operating wavelength may be -20 dB or less.

[0014] It is also acceptable if the bending loss when bent to a diameter of 20 mm at a specified operating wavelength is 1 dB / turn or less.

[0015] It is also acceptable for the mode field diameter to be 5 μm or larger at the specified operating wavelength.

[0016] The outer cladding may be non-circular in cross-section perpendicular to the longitudinal direction.

[0017] The core glass body may have a maximum relative refractive index difference with respect to the refractive index of the outer cladding of 0.18% or more and 0.70% or less, and 1.2% or more and -0.01% or less, a core diameter of 5.0 μm or more and 16.0 μm or less, an inner cladding outer diameter of 17.0 μm or more and 65.0 μm or less, a ratio of the inner cladding outer diameter to the core diameter of 2.0 or more and 6.0 or less, a fiber diameter which is the outer diameter of the outer cladding of 70.0 μm or more and 700.0 μm or less, and a number of cores of 30 or less.

[0018] The core glass body may have a maximum relative refractive index difference of 0.28% or more and 0.51% or less with respect to the refractive index of the outer cladding, a minimum relative refractive index difference of -0.70% or more and -0.03% or less with respect to the refractive index of the outer cladding, a core diameter of 6.5 μm or more and 12.0 μm or less, an inner cladding outer diameter of 29.0 μm or more and 55.0 μm or less, a ratio of the inner cladding outer diameter to the core diameter of 2.5 or more and 5.0 or less, an outer cladding outer diameter (fiber diameter) of 75.0 μm or more and 250.0 μm or less, and a number of cores of 10 or less.

[0019] One aspect of the present invention is a method for manufacturing a multi-core fiber, comprising: an arranging step of arranging a plurality of core glass base materials each having a core and an inner cladding surrounding the outer periphery of the core in a substantially straight line in a direction perpendicular to the longitudinal direction and in such a manner that adjacent core glass base materials contact each other; a forming step of forming an outer cladding surrounding the outer periphery of the arranged plurality of core glass base materials to form a multi-core fiber base material; and a drawing step of heating and melting the formed multi-core fiber base material and drawing the multi-core fiber.

[0020] In the forming step, a glass soot surrounding the outer periphery of the arranged plurality of core glass base materials may be formed by a flame deposition method, and the glass soot may be sintered and vitrified to form the outer cladding.

[0021] The glass soot may be formed using a plurality of burners.

Advantages of the Invention

[0022] According to the present invention, there is an effect that a non-bonded multi-core fiber with high productivity can be realized.

Brief Description of the Drawings

[0023] [Figure 1] FIG. 1 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to Embodiment 1. [Figure 2] FIG. 2 is a diagram showing an example of the refractive index profile of the multi-core fiber according to Embodiment Ⅰ. [Figure 3] FIG. 3 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to Embodiment 2. [Figure 4] FIG. 4 is a diagram showing an example of the refractive index profile of the multi-core fiber according to Embodiment 2. [Figure 5] FIG. 5 is a diagram showing an example of the relationship between the maximum Δ, the core glass body diameter, and the crosstalk between two cores. [Figure 6]Figure 6 shows an example of the relationship between the number of cores, transmission distance, and fiber diameter. [Figure 7] Figure 7 shows an example of the optimal maximum and minimum values ​​of Max Δ depending on the wavelength band. [Figure 8] Figure 8 shows an example of a method for manufacturing a multicore fiber according to Embodiment 1. [Figure 9] Figure 9 shows an example of a method for manufacturing a multicore fiber according to Embodiment 1. [Figure 10] Figure 10 is a schematic cross-sectional view of a multicore fiber according to embodiments 3, 4, 5, and 6 in a plane perpendicular to the longitudinal direction. [Modes for carrying out the invention]

[0024] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below. In each drawing, the same or corresponding components are denoted by the same reference numerals as appropriate, and redundant explanations are omitted as appropriate. Furthermore, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) as defined in ITU-T G.650.1 of the International Telecommunication Union (ITU). In addition, terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0025] (Embodiment 1) Figure 1 is a schematic cross-sectional view of a multicore fiber according to Embodiment 1 in a plane perpendicular to the longitudinal direction. The multicore fiber 10 comprises nine core glass bodies 11, which are a plurality of core glass bodies, and an outer cladding 12.

[0026] The core glass body 11 has a core 11a and an inner cladding 11b surrounding the outer circumference of the core 11a, and is substantially circular in cross-section perpendicular to the longitudinal direction. The core 11a is located approximately in the center of the inner cladding 11b.

[0027] The nine core glass bodies 11 are arranged in a nearly straight line in a cross-section perpendicular to the longitudinal direction, and are positioned so that adjacent core glass bodies 11 are in contact with each other. In other words, the multicore fiber 10 is a single-row type multicore fiber.

[0028] The outer cladding 12 surrounds the outer periphery of the nine core glass bodies 11. The outer cladding 12 has a substantially circular shape in a cross-section perpendicular to the longitudinal direction.

[0029] Next, the relationship between the refractive indices of the core 11a, the inner cladding 11b, and the outer cladding 12 will be explained. Figure 2 shows an example of the refractive index profile of the multicore fiber 10. Figure 2 shows the difference in specific refractive index with respect to the average refractive index of the outer cladding 12 in the radial direction.

[0030] Core 11a contains the portion of the multicore fiber 10 with the highest refractive index. The refractive index of the outer cladding 12 is lower than the maximum refractive index of core 11a.

[0031] The inner cladding 11b, at least in its outer peripheral portion adjacent to the outer cladding 12, has a refractive index lower than that of the outer cladding 12. The inner cladding 11b includes the portion of the multicore fiber 10 with the lowest refractive index. In this case, as shown in Figure 2(a), the inner cladding 11b may have a refractive index higher than that of the outer cladding 12 in its inner peripheral portion adjacent to the core 11a. Alternatively, as shown in Figure 2(b), the inner cladding 11b may have a refractive index lower than that of the outer cladding 12 throughout its radial direction. The refractive index profiles in Figures 2(a) and (b) are also called W-type refractive index profiles. In this case, the inner cladding 11b is also called a depressed layer.

[0032] Examples of constituent materials for the core 11a, inner cladding 11b, and outer cladding 12 are given below. The constituent materials for the core 11a, inner cladding 11b, and outer cladding 12 are not particularly limited as long as they satisfy the refractive index relationship described above. For example, the core 11a is made of quartz glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium. Chlorine is a dopant included in the manufacturing process of the multicore fiber 10 (such as the dehydration process) and may not be intentionally added. Germanium is a dopant that increases the refractive index of the quartz glass. Potassium and sodium are dopants that increase the refractive index of the quartz glass. Fluorine is a dopant that decreases the refractive index of the quartz glass.

[0033] Furthermore, for example, the inner cladding 11b is made of quartz glass, but at least a portion of it (for example, a portion with a refractive index lower than that of the outer cladding 12) is made of fluorine-containing quartz glass.

[0034] Furthermore, for example, the outer cladding 12 consists of pure quartz glass, quartz-based glass that does not contain dopants intentionally added to the outer cladding 12, or quartz-based glass containing fluorine. Quartz-based glass that does not contain intentionally added dopants is, for example, quartz-based glass that contains chlorine included in the manufacturing process or dopants that have diffused from the core 11a or inner cladding 11b during the manufacturing process. Pure quartz glass is extremely high-purity quartz glass with a refractive index of approximately 1.444 at a wavelength of 1550 nm.

[0035] In the multicore fiber 10 configured in this way, adjacent core glass bodies 11 are arranged to be in contact with each other. As a result, as will be described later, gaps are less likely to form between adjacent core glass bodies 11 during manufacturing using methods such as flame deposition. Consequently, the multicore fiber 10 has high manufacturability.

[0036] Furthermore, in the multicore fiber 10, at least the outer peripheral portion of the inner cladding 11b adjacent to the outer cladding 12 has a lower refractive index than the outer cladding 12, so the inter-core XT between adjacent cores 11a can be reduced. As a result, a non-coupled multicore fiber 10 can be suitably realized. Thus, the multicore fiber 10 has, for example, an inter-core XT of -20 dB or less, preferably -30 dB or less, over a length of 10 m at a predetermined operating wavelength. Here, the predetermined operating wavelength is the wavelength of the signal light transmitted when the multicore fiber 10 is used as an optical transmission fiber. It is preferable that the propagation mode of the multicore fiber 10 is single-mode at the operating wavelength.

[0037] Furthermore, since the cores 11a of the multicore fiber 10 are arranged in a straight line, it is easy to input light to the cores 11a using, for example, a ribbon fiber. Therefore, it is also easy to evaluate the optical properties of the multicore fiber 10.

[0038] (Embodiment 2) Figure 3 is a schematic cross-sectional view of a multicore fiber according to Embodiment 2 in a plane perpendicular to the longitudinal direction. The multicore fiber 20 comprises nine core glass bodies 21, which are a plurality of core glass bodies, and an outer cladding 22.

[0039] The core glass body 21 has a core 21a, an inner cladding circumferential portion 21b surrounding the outer circumference of the core 21a, and an inner cladding circumferential portion 21c surrounding the outer circumference of the inner cladding circumferential portion 21b, and is substantially circular in cross-section perpendicular to the longitudinal direction. The inner cladding circumferential portion 21b and the inner cladding circumferential portion 21c constitute the inner cladding. The core 21a is located approximately at the center of the inner cladding.

[0040] The nine core glass bodies 21 are arranged in approximately a single row in a cross-section perpendicular to the longitudinal direction, and are positioned so that adjacent core glass bodies 21 are in contact with each other. In other words, the multicore fiber 20 is a single-row type multicore fiber.

[0041] The outer cladding 22 surrounds the outer periphery of the nine core glass bodies 21. The outer cladding 22 is approximately circular in cross-section perpendicular to the longitudinal direction.

[0042] Next, the relationship between the refractive indices of the core 21a, the inner cladding circumferential portion 21b, the inner cladding circumferential portion 21c, and the outer cladding 22 will be explained. Figure 4 shows an example of the refractive index profile of a multicore fiber 20. Figure 4 shows the difference in specific refractive index with respect to the average refractive index of the outer cladding 22 in the radial direction.

[0043] Core 21a contains the portion of the multicore fiber 20 with the highest refractive index. The refractive index of the outer cladding 22 is lower than the maximum refractive index of core 21a.

[0044] The inner cladding's outer peripheral portion 21c, which is adjacent to the outer cladding 22, has a lower refractive index than the outer cladding 22. The inner cladding includes a portion of the multicore fiber 20 with the lowest refractive index in its outer peripheral portion 21c. In this case, as shown in Figure 4(a), the inner cladding may have a portion of its inner peripheral portion 21b, adjacent to the core 11a, that has a refractive index higher than that of the outer cladding 22. The refractive index profiles in Figures 4(a) and 4(b) are also called trench-type refractive index profiles. In this case, the inner cladding's outer peripheral portion 21c is also called a trench layer.

[0045] The constituent materials of the core 21a, the inner cladding periphery 21b, the inner cladding periphery 21c, and the outer cladding 22 are not particularly limited as long as they satisfy the refractive index relationship described above. For example, the inner cladding periphery 21c is made of quartz glass containing more fluorine than the inner cladding periphery 21b.

[0046] In the multicore fiber 20 configured in this way, the adjacent core glass bodies 21 are arranged to be in contact with each other, similar to the multicore fiber 10, thus offering high manufacturability.

[0047] Furthermore, in the multicore fiber 20, the outer cladding 22 and the outer peripheral portion 21c of the inner cladding adjacent to it have a lower refractive index than the outer cladding 22, thus suitably realizing an uncoupled multicore fiber 10. As a result, the multicore fiber 20 has, for example, a two-core XT of -20 dB or less, preferably -30 dB or less, over a length of 10 m at a predetermined operating wavelength. It is preferable that the propagation mode of the multicore fiber 20 is single-mode at the operating wavelength.

[0048] Furthermore, since the cores 21a of the multicore fiber 20 are arranged in a straight line, it is easy to input light to the cores 21a using, for example, a ribbon fiber. Therefore, it is also easy to evaluate the optical properties of the multicore fiber 20.

[0049] (Suitable properties of multicore fibers) Next, preferred characteristics of the multicore fiber according to the embodiment will be described. The wavelengths used by the multicore fiber belong to the wavelength bands used for optical communication. Such wavelength bands include, for example, the 800-900 nm band, the 1.0 μm band (e.g., 1000 nm-1100 nm), the 1.3 μm band (e.g., 1260 nm-1360 nm), the C-band (e.g., 1530 nm-1565 nm), and the L-band (e.g., 1565 nm-1625 nm).

[0050] The inventors conducted numerous simulations and experiments on the multicore fiber according to the embodiment in each of the above wavelength bands. As a result, it was confirmed that the multicore fiber according to the embodiment can achieve the characteristic that the inter-core XT at the wavelength used and a length of 10 m is -20 dB or less, preferably -30 dB or less.

[0051] Furthermore, the inventors have confirmed that in order to obtain a characteristic where the inter-core XT of 2 cores over a length of 10 m at the wavelength of use is -20 dB or less, it is preferable that the bending loss of the multicore fiber when bent at a diameter of 20 mm at the wavelength of use is 1 dB / turn or less.

[0052] Furthermore, when fusion splicing experiments were conducted on the multicore fibers according to the embodiment, it was confirmed that when the mode field diameter is 5 μm or more at the wavelength used, the average connection loss of the multiple cores can be reduced to 0.5 dB or less, which is preferable.

[0053] Furthermore, it was confirmed that, in order to achieve a 2-core XT of -20dB or less, a bending loss of 1dB / turn or less when bent at a diameter of 20mm, and a mode field diameter of 5μm or more at the wavelength of use, the following structural parameters are preferable.

[0054] In other words, it is preferable that the structural parameters of the multicore fiber are such that the maximum value of the relative refractive index difference between the core glass body and the outer cladding relative to the refractive index is 0.18% or more and 0.70% or less, the minimum value of the relative refractive index difference between the core glass body and the outer cladding relative to the refractive index is -1.2% or more and -0.01% or less, the core diameter is 5.0 μm or more and 16.0 μm or less, the outer diameter of the inner cladding is 17.0 μm or more and 65.0 μm or less, the ratio of the outer diameter of the inner cladding to the core diameter is 2.0 or more and 6.0 or less, and the fiber diameter, which is the outer diameter of the outer cladding, is 70.0 μm or more and 700.0 μm or less.

[0055] Furthermore, the maximum value of the relative refractive index difference between the core glass body and the outer cladding is the relative refractive index difference between the maximum refractive index in the core and the outer cladding, and may be referred to as maximum Δ below. Also, the minimum value of the relative refractive index difference between the core glass body and the outer cladding is the relative refractive index difference between the minimum refractive index in the inner cladding and the outer cladding, and may be referred to as minimum Δ below.

[0056] Furthermore, it is more preferable that the structural parameters of the multicore fiber are such that the maximum value (maximum Δ) of the relative refractive index difference between the core glass body and the outer cladding is 0.28% or more and 0.51% or less, the minimum value (minimum Δ) of the relative refractive index difference between the core glass body and the outer cladding is -0.70% or more and -0.03% or less, the core diameter is 6.5 μm or more and 12.0 μm or less, the outer diameter of the inner cladding is 29.0 μm or more and 55.0 μm or less, the ratio of the outer diameter of the inner cladding to the core diameter is 2.5 or more and 5.0 or less, and the fiber diameter, which is the outer diameter of the outer cladding, is 75.0 μm or more and 250.0 μm or less.

[0057] However, it is preferable to select an even more optimal range of structural parameters from the range described above, depending on the wavelength band used.

[0058] For example, let's discuss the setting of structural parameters when the wavelength used belongs to the C-band. Figure 5 shows an example of the relationship between the maximum Δ and core glass diameter and the inter-core XT (XT in the figure) obtained by simulation calculations. In Figure 5, the minimum Δ is fixed at -2.5%, and the core diameter is optimized so that the effective cutoff wavelength is 1450 nm. Also, in Figure 5, the wavelength used is 1550 nm.

[0059] As shown in Figure 5, the inter-core XT is 10dB to 20dB in the lower left region of the graph, but gradually decreases as the graph moves towards the upper right, reaching -120dB to -110dB in the upper right region. In the figure, line L1 indicates that the inter-core XT is -20dB, and in the region above line L1, the inter-core XT is less than -20dB. Also, in the figure, line L2 indicates that the bending loss when bent at a diameter of 20mm is 1dB / turn, and in the region to the right of line L2, the bending loss when bent at a diameter of 20mm is less than 1dB / turn. Furthermore, in the figure, line L3 indicates that the transmission loss is 0.5dB, and in the region to the left of line L3, the transmission loss is less than 0.5dB.

[0060] As shown in Figure 5, the maximum Δ and core glass diameter can be set to an optimal combination depending on the required bending loss, transmission loss, and XT between the two cores. Furthermore, since increasing the maximum Δ increases transmission loss and decreases the mode field diameter, it is preferable to design the structural parameters while also considering the connection characteristics.

[0061] Furthermore, while increasing the diameter of the core glass body is desirable for reducing the XT between two cores, it increases the fiber diameter (outer diameter of the outer cladding). Also, if one attempts to reduce the XT between two cores while keeping the fiber diameter constant, the number of core glass bodies (i.e., the number of cores) included in the multicore fiber decreases. Therefore, when setting structural parameters, it is important to determine the target transmission distance (length of the multicore fiber), the number of cores, and the fiber diameter.

[0062] Figure 6 shows an example of the relationship between the number of cores, transmission distance, and fiber diameter. In Figure 6, the structural parameters are set so that a -30 dB XT is obtained between two cores over the transmission distance (length of the multicore fiber). Also, in Figure 6, the wavelength used is 1550 nm.

[0063] As shown in Figure 6, the fiber diameter is 100 μm to 120 μm in the lower left region of the graph, but gradually increases as you move towards the upper right of the graph, reaching 520 μm to 540 μm in the upper right region.

[0064] Figure 6 shows that, for example, if the target fiber diameter is 160 μm, the number of cores is 6 for a transmission distance of 10 m, 5 for a transmission distance of 1 km, and 4 for a transmission distance of 50 km. Furthermore, if the transmission distance is 100 m and the number of cores is to be 8, the fiber diameter needs to be increased to approximately 250 μm. Also, if the fiber diameter is a typical 125 μm, the number of cores is limited to approximately 4 for a transmission distance of 100 m.

[0065] Thus, the number of cores that can be achieved in a given fiber diameter depends on the wavelength used, the transmission distance, and other factors. Therefore, if you want to achieve a desired number of cores, you should set the fiber diameter according to the wavelength used, the transmission distance, and other factors. The number of cores is not particularly limited, but for example, 30 or less is preferred, and 10 or less is more preferred.

[0066] As will be described later, in the multicore fiber according to the embodiment, the outer cladding may be non-circular in a cross-section perpendicular to the longitudinal direction. In this case, the fiber diameter can be defined as the length of the longest part in a cross-section perpendicular to the longitudinal direction of the outer cladding.

[0067] Furthermore, the optimal range of structural parameters depends on the wavelength used, so it is preferable to set appropriate structural parameters according to the wavelength used. For example, Figure 7 shows an example of the optimal maximum and minimum values ​​of maximum Δ according to the wavelength band. Note that the optimal maximum and minimum values ​​are optimal values ​​from the viewpoint of obtaining the characteristics of single-mode propagation, a two-core XT of -20 dB or less at a length of 10 m, a bending loss of 1 dB / turn or less when bent at a diameter of 20 mm, and a mode field diameter of 5 μm or more. As shown in Figure 7, the optimal minimum value, optimal maximum value, and range of maximum Δ differ depending on the wavelength band and tend to increase as the wavelength band lengthens.

[0068] Furthermore, Table 1 shows the preferred range of structural parameters for each wavelength band, as obtained by the inventors through systematic studies. This preferred range is suitable from the viewpoint of obtaining the following characteristics: single-mode propagation, a two-core XT of -20 dB or less at a length of 10 m, a bending loss of 1 dB / turn or less when bent at a diameter of 20 mm, and a mode field diameter of 5 μm or more. As shown in Table 1, by using an appropriate combination of structural parameters according to the wavelength band, suitable characteristics for a single-row, uncoupled multicore fiber can be obtained. The number of cores is not particularly limited, but from the viewpoint of suppressing an excessive increase in fiber diameter, it is preferable that the number of cores be between 4 and 10 in any wavelength band. [Table 1]

[0069] (Manufacturing method) An example of a method for manufacturing multicore fibers according to the embodiment will be described with reference to Figures 8 and 9. The method for manufacturing multicore fibers according to this example is an example of a method for manufacturing multicore fibers according to Embodiment 1, and comprises at least a placement step, a formation step, and a drawing step.

[0070] In the arrangement process, as shown in Figure 8(a), nine core glass base materials 110 are arranged in a nearly straight line perpendicular to the longitudinal direction, and adjacent core glass base materials 110 are placed in contact with each other. The core glass base materials 110 are then fixed in place with support jigs or the like. Here, the longitudinal direction is the direction perpendicular to the drawing, and the direction perpendicular to the longitudinal direction is the left-right direction of the drawing.

[0071] The core glass matrix 110 has a core 110a and an inner cladding 110b surrounding the outer circumference of the core 110a, and is substantially circular in cross-section perpendicular to the longitudinal direction. The core glass matrix 110 is the portion that will become the core glass body 11 of the multicore fiber 10. The core 110a is the portion that will become the core 11a, and the inner cladding 110b is the portion that will become the inner cladding 11b. Such a core glass matrix 110 can be manufactured in one piece based on vapor-phase methods such as the VAD (Vapor-phase Axial Deposition) method.

[0072] In the formation process, an outer cladding is formed to surround the outer periphery of the nine arranged core glass base materials 110, thereby forming a multicore fiber base material. In the formation process of this example, a glass soot surrounding the outer periphery of the nine arranged core glass base materials 110 is formed by flame deposition, and this glass soot is sintered and vitrified to form the outer cladding.

[0073] Specifically, as shown in Figures 8(a) to 8(c), two burners B1 and B2 for performing the flame deposition method (for example, the OVD (Outside Vapor Deposition) method) are positioned so as to sandwich nine core glass base materials 110 from the top and bottom directions in the drawing. Then, glass soot 120 is deposited onto the core glass base materials 110 while moving the burners B1 and B2 from one end to the second end in the arrangement direction of the core glass base materials 110 (left and right direction in the drawing). When the burners B1 and B2 reach a predetermined position on the second end, the burners B1 and B2 are moved to the second end, that is, the burners B1 and B2 are moved back and forth in the arrangement direction of the core glass base materials 110.

[0074] Next, as shown in Figure 9(a), burner B1 is moved back and forth so that the reciprocating distance gradually decreases, as indicated by the arrow Ar. Although only burner B1 is shown in Figure 9(a), burner B2 is also moved back and forth in the same way as burner B1. As a result, the glass soot 120 is deposited such that the central part in the arrangement direction of the core glass base material 110 bulges in a cross section perpendicular to the longitudinal direction, and becomes nearly circular. To make the shape closer to circular, the reciprocating speed of burners B1 and B2 and the amount of glass soot 120 deposited per unit time may be adjusted.

[0075] Next, as shown in Figure 9(b), the intermediate, which is a core glass matrix 110 with glass soot 120 deposited on it, is rotated around an axis perpendicular to its longitudinal direction, and more glass soot 120 is deposited using burners B1 and B2 (not shown in Figure 9(b)). At this time, by adjusting the rotation speed and the amount of glass soot 120 deposited per unit time, the shape of the glass soot 120 is made to be even closer to a circular shape in a cross section perpendicular to its longitudinal direction.

[0076] Subsequently, the glass soot 120 is sintered and vitrified to form the outer cladding of the multicore fiber matrix. This completes the formation of the multicore fiber matrix.

[0077] In the wire drawing process, the multicore fiber base material is heated and melted using a known wire drawing device, and then wire drawing is performed. This makes it possible to manufacture the multicore fiber 10.

[0078] In the above manufacturing method, since adjacent core glass base materials 110 are arranged in contact with each other, it is difficult for gaps (voids) to form between the two core glass base materials 110 where glass soot 120 does not deposit. Such gaps are difficult to fill by subsequent deposition of glass soot 120 and may affect the properties of the multicore fiber, thus potentially leading to a decrease in manufacturability. Therefore, making it difficult for gaps to form, as in the above manufacturing method, is effective from the standpoint of manufacturability. Furthermore, preventing the formation of gaps altogether is even more effective.

[0079] Furthermore, in the above manufacturing method, the glass soot 120 is formed using burners B1 and B2. By using multiple burners in this way, the manufacturing time can be shortened, and the manufacturing cost of the multicore fiber 10 can be reduced. However, the number of burners is not particularly limited; it may be one or three or more.

[0080] (Examples) A multicore fiber of the example was fabricated according to the manufacturing method described above. The multicore fiber of the example has a configuration in which the number of core glass bodies is 4, as in the multicore fiber 20 of Embodiment 2. The core glass matrix was fabricated using the VAD method.

[0081] The refractive index profile of the multicore fiber in the example had the shape shown in Figure 4(b). The structural parameters of the multicore fiber in the example are as follows: The maximum Δ is 0.38%, and the minimum Δ is -0.25%. The core radius is 4.1 μm (i.e., the diameter is 8.2 μm). The outer diameter of the inner cladding (i.e., the outer diameter of the core glass body) is 24 μm. The ratio of the outer diameter of the inner cladding to the diameter of the core is 2.9. The fiber diameter is 125 μm.

[0082] Table 2 shows the optical properties obtained by measuring the multicore fiber of the example. In Table 2, the optical properties include the zero-dispersion wavelength λ0, dispersion slope at the zero-dispersion wavelength, mode field diameter (MFD) at a wavelength of 1310 nm, cutoff wavelength λcc, transmission loss at a wavelength of 1550 nm, bending loss when bent to a diameter of 20 mm at a wavelength of 1550 nm, and the inter-core XT at a wavelength of 1550 nm and a length of 10 m. The four cores are numbered from the end as core 1, core 2, core 3, core 4, and so on. In the inter-core XT, for example, (1-2) refers to the inter-core XT between core 1 and core 2.

[0083] The optical properties shown in Table 2 are suitable for all four cores: Core 1, Core 2, Core 3, and Core 4. For example, as can be seen from the λcc value, the propagation mode is single-mode at a wavelength of 1310 nm for all cores. Therefore, the propagation mode is also single-mode at a wavelength of 1550 nm for all cores.

[0084] Furthermore, for example, the inter-core XT at a wavelength of 1550 nm and a length of 10 m is -30 dB or less between any two cores.

[0085] Furthermore, the mode field diameters of core 1, core 2, core 3, and core 4 at a wavelength of 1550 nm were 9.73 μm, 9.71 μm, 9.75 μm, and 9.72 μm, respectively, all of which were 5 μm or larger. Therefore, all of the multicore fibers in the example, with a wavelength of 1550 nm, satisfied the following characteristics for single-mode propagation, a two-core XT of -20 dB or less over a length of 10 m, a bending loss of 1 dB / turn or less when bent at a diameter of 20 mm, and a mode field diameter of 5 μm or larger.

[0086] Furthermore, when the XT between two cores was measured at a wavelength of 1310 nm for the multicore fiber in the example, a value of -30 dB or less was obtained for a length of 100 m. [Table 2]

[0087] (Other embodiments) Figure 10 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of a multicore fiber according to embodiments 3, 4, 5, and 6, which represent other embodiments. In these multicore fibers, the outer cladding is non-circular in a cross-section perpendicular to the longitudinal direction.

[0088] Figure 10(a) is a diagram of a multicore fiber 30 according to Embodiment 3. The multicore fiber 30 comprises nine core glass bodies 11 and an outer cladding 32. The multicore fiber 30 has a configuration in which the outer cladding 12 is replaced with the outer cladding 32 in the multicore fiber 10 according to Embodiment 1 shown in Figure 1.

[0089] The outer cladding 32 is identical to the outer cladding 12 except that its cross-section perpendicular to the longitudinal direction is elliptical, so a redundant explanation will be omitted. Such multicore fibers 30, like the multicore fibers 10, are uncoupled multicore fibers that are highly manufacturable. For example, the elliptical shape of the outer cladding 32 can be achieved in the example manufacturing method described above by adjusting the reciprocating speed of the burner, the rotation speed of the core glass matrix, and the amount of glass soot deposited per unit time.

[0090] Figure 10(b) is a diagram of a multicore fiber 40 according to Embodiment 4. The multicore fiber 40 comprises nine core glass bodies 11 and an outer cladding 42. The multicore fiber 40 has a configuration in which the outer cladding 12 of the multicore fiber 10 is replaced by the outer cladding 42.

[0091] The outer cladding 42 is the same as the outer cladding 12 except that its cross-section perpendicular to the longitudinal direction is disc-shaped, so a redundant explanation will be omitted. Such multicore fibers 40, like the multicore fibers 10, are uncoupled multicore fibers that are easy to manufacture. For example, the disc shape of the outer cladding 42 can be achieved in the example manufacturing method described above by adjusting the reciprocating speed of the burner, the rotation speed of the core glass matrix, and the amount of glass soot deposited per unit time.

[0092] Figure 10(c) is a diagram of a multicore fiber 50 according to Embodiment 5. The multicore fiber 50 comprises nine core glass bodies 11 and an outer cladding 52. The multicore fiber 50 has a configuration in which the outer cladding 12 of the multicore fiber 10 is replaced by the outer cladding 52.

[0093] The outer cladding 52 is the same as the outer cladding 12 except that its cross-section perpendicular to the longitudinal direction is rectangular, so a redundant explanation will be omitted. Such multicore fibers 50, like the multicore fibers 10, are uncoupled multicore fibers that are easy to manufacture. For example, the rectangular shape of the outer cladding 42 can be achieved by the reciprocating movement of the burner as shown in Figure 8, without adjusting the reciprocating movement distance of the burner or rotating the core glass base material as shown in Figure 9, in the example of the manufacturing method described above.

[0094] Figure 10(d) shows a multicore fiber 60 according to Embodiment 6. The multicore fiber 60 has a configuration in which two coating layers 63 and 64 are provided so as to surround the outer periphery of the outer cladding 52 of the multicore fiber 50 according to Embodiment 5. The coating layers 63 and 64 are made of resin. This resin is, for example, an ultraviolet curing resin, but is not particularly limited as long as it is a resin used as a coating for optical fibers. By providing the coating layers 63 and 64 in this way, the glass portion of the multicore fiber 60 is protected. Note that the coating layers are not limited to two layers, but may be one layer or three or more layers.

[0095] Furthermore, coating layers such as coating layers 63 and 64 may be provided on multicore fibers according to other embodiments 1 to 4. Even when the outer cladding is non-circular in a cross-section perpendicular to the longitudinal direction, a suitable coating layer can be formed by optimizing the coating process.

[0096] It should be noted that the present invention is not limited to the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Explanation of Symbols]

[0097] 10, 20, 30, 40, 50, 60: Multicore fiber 11, 21: Core glass body 11a, 110a: Core 11b, 110b: Inner cladding 12, 22, 32, 42, 52: Outer cladding 21a: Core 21b: Inner cladding inner circumference 21c: Outer perimeter of inner cladding 63, 64: Covering layer 110: Core glass base material 120: Glass Suit Ar: Arrow B1, B2: Burner

Claims

1. A plurality of core glass bodies having a core and an inner cladding surrounding the outer circumference of the core, An outer cladding surrounding the outer periphery of the plurality of core glass bodies, Equipped with, The plurality of core glass bodies are arranged in a substantially straight line in a cross section perpendicular to the longitudinal direction, and adjacent core glass bodies are in contact with each other. The refractive index of the outer cladding is lower than the maximum refractive index of the core. At least the outer peripheral portion of the inner cladding adjacent to the outer cladding has a refractive index lower than that of the outer cladding. The rows of the aforementioned core glass bodies are arranged so as to pass through the center of the outer cladding in the cross-section. The number of the aforementioned multiple core glass bodies is four or more. At a given operating wavelength, the propagation mode is single-mode. The plurality of core glass bodies have a W-type refractive index profile. Multicore fiber.

2. At least a portion of the inner cladding is made of fluorine-containing quartz glass. The multicore fiber according to claim 1.

3. The outer cladding consists of pure quartz glass, quartz-based glass without intentionally added dopants, or quartz-based glass containing fluorine. The multicore fiber according to claim 1.

4. The core is made of quartz glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium. The multicore fiber according to claim 1.

5. The intercore crosstalk over a length of 10 m at the specified operating wavelength is -20 dB or less. The multicore fiber according to claim 1.

6. The bending loss when bent to a diameter of 20 mm at a predetermined operating wavelength is 1 dB / turn or less. The multicore fiber according to claim 1.

7. The mode field diameter is 5 μm or larger at the specified operating wavelength. The multicore fiber according to claim 1.

8. The outer cladding is non-circular in a cross-section perpendicular to the longitudinal direction. The multicore fiber according to claim 1.

9. The maximum value of the relative refractive index difference between the core glass body and the outer cladding is 0.18% or more and 0.70% or less. The minimum value of the relative refractive index difference between the core glass body and the outer cladding is -1.2% or more and -0.01% or less. The diameter of the core is 5.0 μm or more and 16.0 μm or less. The outer diameter of the inner cladding is 17.0 μm or more and 65.0 μm or less. The ratio of the outer diameter of the inner cladding to the diameter of the core is 2.0 or more and 6.0 or less. The outer diameter of the outer cladding, which is the fiber diameter, is 70.0 μm or more and 700.0 μm or less, and the number of cores is 30 or less. The multicore fiber according to claim 1.

10. The maximum value of the relative refractive index difference between the core glass body and the outer cladding is 0.28% or more and 0.51% or less. The minimum value of the relative refractive index difference between the core glass body and the outer cladding is -0.70% or more and -0.03% or less. The diameter of the core is 6.5 μm or more and 12.0 μm or less. The outer diameter of the inner cladding is 29.0 μm or more and 55.0 μm or less. The ratio of the outer diameter of the inner cladding to the diameter of the core is 2.5 or more and 5.0 or less. The outer diameter of the outer cladding, which is the fiber diameter, is 75.0 μm or more and 250.0 μm or less, and the number of cores is 10 or less. The multicore fiber according to claim 1.

11. A method for manufacturing a multicore fiber according to claim 1, A sorting step involves arranging a plurality of core glass base materials, each having a core and an inner cladding surrounding the outer circumference of the core, in a substantially straight line perpendicular to the longitudinal direction, such that adjacent core glass base materials are in contact with each other. A forming step to form a multicore fiber base material by forming an outer cladding surrounding the outer periphery of the multiple core glass base materials arranged as described above, The multicore fiber base material formed above is heated and melted, and the multicore fiber is drawn in a line drawing step, A method for manufacturing multicore fibers.

12. In the formation process, a glass soot surrounding the outer periphery of the arranged multiple core glass base materials is formed by flame deposition, and the glass soot is sintered and vitrified to form the outer cladding. A method for producing a multicore fiber according to claim 11.

13. Forming the glass soot using multiple burners A method for producing a multicore fiber according to claim 12.

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